The main advantage of optical correlators, as compared with their digital counterparts, is that high-resolution Fourier transform operation on the input optical images may be rapidly executed, typically in nanoseconds by simply transmitting the input image through a single lens. However, the overall speed of an optical correlator is still limited by how fast the information can be updated on the input devices (spatial light modulators), the real-time holographic material, and the output device (camera or detector array). The speeds of these three components are equally important because the slowest component will determine the overall speed of the system.
Gregory Gheen and Li-Jen Cheng have reported using photorefractive GaAs in a paper titled "Optical correlators with fast updating speed using photorefractive semiconductor materials," Applied Optics, Vol. 27, No. 13, pp. 2756-2761, (1988). The objective was to improve the speed of real-time optical correlators. In principle, all nonlinear optical materials can be used as real-time holographic materials. However, although many nonlinear optical materials have very short response times, they require too high an intensity to yield a practical diffraction efficiency.
Photorefractive crystals are in general slower than other nonlinear optical materials, but they can operate with a much lower power requirement. Among all the photorefractive crystals, semiconductors such as GaAs, InP, and CdTe are in general one to two orders of magnitude faster than photorefractive oxides such as BaTiO.sub.3, SBN and BSO. They are, therefore, more suitable for real-time applications.
In the GaAs based optical correlator reported by Gregory Gheen and Li-Jen Cheng, the input image and the reference image were put on photographic films only. That does not lend itself to real-time image correlation. What is required is some means, such as a liquid crystal TV panel having a thin-film transistor active matrix of MxN pixels where M and N are integers, to serve the function of spatial light modulators to produce in real time input and reference images. Consequently, an object of this invention is to provide a real-time GaAs-based VanderLugt optical correlator in which real-time input devices, i.e., liquid-crystal TVs (LCTVs), are used. The output device is a vidicon camera. The speeds of both the LCTVs and the vidicon camera are video rate, while that of the GaAs may be much higher (as high as 1000 frames/sec). Therefore, the speed bottleneck of the optical correlator is at both the input and the output devices.
When the shape, size and orientation of the object in the input image and the object in the reference image are the same, the correlator displays a bright spot (autocorrelation peak) in the output image at an equivalent location of the object in the input image. Therefore the autocorrelation peak can be used not only to identify an object but also to track its location. Edge-enhanced input and reference images yield a sharper autocorrelation peak and thus a better defined position of the object in the input image. Nevertheless, the autocorrelation peak intensity of edge enhanced input and reference images is more sensitive to the relative size and orientation of the object in the input image with respect to the object in the reference image.
Edge enhancement of an image can be implemented by using the dependence of the diffraction efficiency (or modulation depth) on the write-beam ratio in four-wave mixing in the photorefractive crystal. J. P. Huignard and J. P. Herriau, "Real-time coherent object edge reconstruction with Bi.sub.12 SiO.sub.20 crystals," Appl. Opt., Vol. 17, No. 17, pp. 2671-2672 (1978); J. Feinberg, "Real-time edge enhancement using the photorefractive effect," Opt. Lett., Vol. 5, pp. 330-332 (1980); and E. Ochoa, L. Hesselink and J. W. Goodman, "Real-time intensity inversion using two-wave and four-wave mixing photorefractive Bi.sub.12 GeO.sub.20," Appl. Opt., Vol. 24, pp. 1826-1832, (1985). The technique of real-time coherent object edge reconstruction with Bi.sub.12 SiO.sub.20 reported by J. P. Huignard and J. P. Herriau can be used to edge-enhance the input image, but it cannot be used to edge-enhance the reference image. Consequently, another object of this invention is to implement edge enhancement on the reference image using the dependence of the index grating erasing, which reduces the diffraction efficiency on the read beam intensity.